Targeted transcranial direct current stimulation for rehabilitation after stroke

Abstract

Transcranial direct current stimulation (tDCS) is being investigated as an adjunctive technique to behavioral rehabilitation treatment after stroke. The conventional "dosage", consisting of a large (25 cm(2)) anode over the target with the cathode over the contralateral hemisphere, has been previously shown to yield broadly distributed electric fields whose intensities at the target region are less than maximal. Here, we report the results of a systematic targeting procedure with small "high-definition" electrodes that was used in preparation for a pilot study on 8 stroke patients with chronic aphasia. We employ functional and anatomical magnetic resonance imagery (fMRI/MRI) to define a target and optimize (with respect to the electric field magnitude at the target) the electrode configuration, respectively, and demonstrate that electric field strengths in targeted cortex can be substantially increased (63%) over the conventional approach. The optimal montage exhibits significant variation across subjects as well as when perturbing the target location within a subject. However, for each displacement of the target co-ordinates, the algorithm is able to determine a montage which delivers a consistent amount of current to that location. These results demonstrate that MRI-based models of current flow yield maximal stimulation of target structures, and as such, may aid in reliably assessing the efficacy of tDCS in neuro-rehabilitation.

Copyright © 2013 Elsevier Inc. All rights reserved.

Figures

Figure 1

The optimized electrode montages for all subjects. Left: Anatomical MRI depicting lesion anatomy and the location of the peri-lesional target (orange circle). Right: optimal montages with anodes (cathodes) marked in red (black). The approximate location of the target, projected onto the scalp, is indicated with an ‘x’ and resides in the left temporal lobe for all aphasic subjects. The locations of both the anodes and cathodes are non-trivial, subject-dependent, and thus cannot be intuitively predicted from target location alone.

Figure 2

Conventional (1st and 3rd columns) and optimized (2nd and 4th columns) electric fields for all subjects. The direction (size) of the overlaid arrows depict the direction (magnitude) of the attained electric field. With the conventional design, the large electrode area and idiosyncratic head anatomy lead to moderate electric field intensities at the target (open circle). Meanwhile, the optimized design steers the applied current to the target, resulting in a marked increase in the electric field strength at the target.

Figure 3

Evaluating the sensitivity of montage optimization. The depicted montages are the optimal solutions averaged across all nodes in a 3 mm vicinity of the physiological target. The values listed above each averaged montage refers to the corresponding electric field intensities (mean ± standard deviation) achieved in the 3 mm neighborhood of probed targets. Electrodes whose applied currents are close to ± 1mA are reliably selected regardless of the precise target location. Values close to zero indicate that this location is seldom selected. Small variations of the target node have a marked effect on the location of the optimally selected electrodes. However, the resulting electric field magnitude at the target node is robust to these movements of the optimal montage.

Figure 4

Conventional versus optimized tDCS: (A) Electric field magnitudes achieved at the target during conventional and optimized stimulation: the optimized design delivers 64% more current (averaged across subjects). (B) Accuracy (relative to baseline) on a word-naming task after conventional and optimized tDCS. Subjects improve by 8 % after the conventional and by 11 % after the optimized treatment. Due to the large intersubject variability in the responses to optimized tDCS, this behavioral improvement was not found to be statistically significant.